FIELD OF THE INVENTION
The invention relates to a method for tank venting in an internal combustion engine having a lambda control device for controlling an air/fuel mixture to be fed to the internal combustion engine, which includes intermediately storing fuel vapors arising in a fuel supply container in an adsorption container, preferably having an activated carbon filter, the activated carbon filter exhibiting a degree of loading dependent on the mass of fuel vapors adsorbed; initiating a flushing procedure of the activated carbon filter within the context of a regeneration phase, in selected operating regions of the internal combustion engine; continuously varying a passage cross section of a tank venting valve which is connected between the adsorption container and the internal combustion engine and can be driven by an electrical signal with a variable pulse duty factor; and introducing a flushing stream into the intake tract of the internal combustion engine, downstream of a throttle, for feeding to combustion.
The purpose of such tank venting installations, which are primarily used in motor vehicles, is to avoid the evaporation of hydrocarbons from the fuel tank into the atmosphere.
To that end, the tank venting installation generally has a fuel tank and a tank venting valve, which is connected to the intake tract of an internal combustion engine which drives the motor vehicle. As a result, with the aid of a negative pressure in the suction pipe, fuel vapors are sucked up and fed to the combustion in the cylinders of the internal combustion engine. It is normal for the volume located above the fuel in the fuel tank not to be sucked up directly, but for the fuel vapor to be intermediately stored in a separate container, which contains an adsorbent material, as a rule an activated carbon filter. That prevents the emergence of the fuel vapor into the environment. The activated carbon filter adsorbs fuel vapors in those periods in which no suction is being performed by the suction pipe, for example when the internal combustion engine is stopped or the tank venting valve is kept closed on the basis of the current operating state of the internal combustion engine.
Since the activated carbon filter can only store a limited fuel mass, it must be flushed in suitable operating ranges of the internal combustion engine. In that case, the tank venting valve, which is disposed in a line between the activated carbon filter and the suction pipe of the internal combustion engine, is opened by being driven through the use of suitable signals from an electronic control device of the internal combustion engine. The opening cross section of the tank venting valve and therefore the flushing stream of the activated carbon filter can be set through the use of a variation of the driving pulse duty factor of that signal.
However, the flushing rate cannot be selected to be arbitrarily high, since otherwise the lambda controller of the lambda control device reaches a limit (controller stop), and a predefined air/fuel ratio can no longer be adjusted.
German Published, Non-Prosecuted Patent Application DE 38 13 220 A1, corresponding to U.S. Pat. No. 5,072,712, has disclosed a method and a device for setting a tank venting valve. In order to obtain actuating values for driving the tank venting valve, which is connected to the intake connection of an internal combustion engine, use is made of a control factor which is supplied by a lambda controller calculation step. The control factor modifies a charging factor until a regeneration fuel quantity which leads to no deviation from the lambda desired value is discharged through the tank venting valve. The controlled charging factor modifies pilot control values for the regeneration fuel quantity which may be supplied in the case of an operating state that is present at that time. The maximum possible gas stream through the tank venting valve is calculated as a function of the pressure relationships at the tank venting valve that prevail in the case of the respective operating state. The device contains a flow determining device to take into account the pressure relationships at the tank venting valve, and a charging control device to adapt a provisionally assumed charging factor to the charging factor which is actually present.